This invention relates to methods and apparatus for forming trenches in beds located under water and laying pipeline in the trenches. Typical prior art structures and methods for doing this are shown in U.S. Pat. No. 2,144,063. According to the invention, it is desired to provide a method of laying pipeline in a bed substantially under water that may be utilized both in clay, sand, and mixtures of clay and sand, and can be done in a high speed manner with a minimum of accessory equipment. The pipeline must of course be layed along a predetermined desired line. According to the present invention is it also desired to provide apparatus that is relatively simple and uncomplicated, yet can effectively form a trench along a predetermined path and lay the pipe in the trench in a manner in which it will be advantageously disposed.
According to the present invention, a method of laying pipeline in a bed substantially under water is provided comprising the steps of disposing the pipe on the bed along the general line the pipeline will take, and excavating a trench in the bed generally parallel to the pipe by transporting spoil to the opposite side of the trench as the pipe and by excavating sufficient material adjacent the pipe so that the pipe will be supported by the bed after initial excavation but so that the bed ultimately will shear along a shear line under the weight of the pipe alone. In this way, the pipe drops into the trench, along with material of the bed above the shear line, in a simple and efficient manner after trenching. The excavating step is advantageously accomplished by forming a trench deeper than the diameter of the pipe that is trapezoidal in cross-section, with top, bottom, near wall, and far wall sides. The top and bottom sides are generally parallel and the near wall side -- adjacent the pipe -- is generally perpendicular to the top and bottom. The shear line is disposed at substantially the same angle with respect to the top side as the far side, although this angle can be greater. Material that has fallen into the trench after collapse of the pipe into the trench may be shifted out of the trench.
According to the present invention, a post trenching plow is provided comprising a frame member with a tandem set of wheels mounted to a forward portion of the frame member, a center line of the plow extending between the wheels posterior thereof. A first share is mounted to the frame member posterior of the wheels and located asymmetrically with respect to the center line. Means are provided for depositing the spoil on the opposite side of the center line as the share, and for counterbalancing the eccentric reaction on the share during trenching due to the asymmetrical mounting of the share, so that the plow may move in a straight line. The counterbalancing means and the depositing means may be provided by the same member -- a mouldboard extending from the share to the opposite side of the center line as the share. Means for guiding the movement of the excavating means during trenching so that the trench is formed along predetermined lines also is preferably provided, including a plurality of guide rollers operatively mounted to the frame on the same side of the center line as the share, and rotatable about generally vertical axes, a pair of guide rollers being provided in each set with the surfaces of the rollers in each set being spaced apart a distance corresponding to the diameter of the pipe, or a cable or a like guiding structure, for engaging the guide member during trenching. A second (or more) share is also provided mounted posterior of the first share, also located asymmetrically with respect to the center line, for shifting material that collapsed into the trench with the pipe out of the trench.
It is the primary objects of the present invention to provide an advantageous method and apparatus for forming a trench in an under water bed and laying a pipeline in the trench. This and other objects of the invention will become clear from an inspection of the Detailed Description of the Invention, and from the appended claims.